1. Field of the Invention
The present invention relates to a Synchronization CHannel (SCH) of a broadband wireless communication system. More particularly, the present invention relates to an apparatus and method for transmitting/receiving a Secondary SCH (S-SCH) for distinguishing a cell IDentification (IDcell) in an Institute of Electrical and Electronics Engineers (IEEE) 802.16m wireless communication system.
2. Description of the Related Art
Many wireless communication technologies have been proposed as candidates of a high speed mobile communication. Among these, an Orthogonal Frequency Division Multiplexing (OFDM) technology is now recognized as the leading next-generation wireless communication technology. In the future, it is expected that the OFDM technology will be used in most of the wireless communication technologies. At present, even the IEEE 802.16 Wireless Metropolitan Area Network (WMAN), which is referred to as a 3.5-Generation (3.5G) technology, adopts the OFDM technology as its standard.
The OFDM scheme is a scheme of transmitting data using a multi-carrier. Namely, the OFDM scheme is a type of Multi Carrier Modulation (MCM) scheme of parallel converting symbol streams input in series and modulating each of the symbol streams into a plurality of sub-carriers having cross orthogonality, i.e., a plurality of sub-channels for transmission.
In a system using the OFDM scheme, a Base Station (BS) transmits an SCH to a Mobile Station (MS) for the sake of timing synchronization and BS distinguishment. Accordingly, the MS may distinguish the BS to which the MS belongs using the SCH. A position where the SCH is transmitted is predefined between a transmitter and a receiver. As a result, the SCH operates as a kind of reference signal.
As a design method for the SCH, a variety of methods may be used, but the most noticed method is currently a method of loading and transmitting a Pseudo-Random (PR) sequence native to a BS on subcarriers at preset intervals in a frequency domain. In the case of mapping a sequence at preset intervals without loading and transmitting a sequence on all subcarriers, regarding a time domain signal after an Inverse Fast Fourier Transform (IFFT) operation, it may be identified that a repetition of a constant pattern takes place within an OFDM symbol. At this time, the repetition count is varied depending on the sequence mapping interval of the frequency domain.
An SCH used in an IEEE 802.16e system of the related art is described below.
FIG. 1 is a diagram illustrating a frequency domain signal of an SCH according to the related art.
Referring to FIG. 1, in the SCH of the related art, a sequence value is allocated at an interval of every third subcarrier in the frequency domain.
A time domain signal of an SCH corresponding to that of FIG. 1 is illustrated in FIG. 2.
FIG. 2 is a diagram illustrating a time domain signal of an SCH according to the related art.
Referring to FIG. 2, the SCH of the related art has a format in which the same signal is repeated 3 times in a time domain. An MS acquires timing synchronization using a repetition pattern of the SCH. At this time, a size of IFFT is equal to the power of ‘2’ but ‘3’ (repetition count) is not equal to a divisor of the IFFT size and therefore, the three-time repetition pattern is not a complete repetition pattern but an incomplete repetition pattern. Accordingly, in case that the MS is positioned at a cell boundary or cell edge of a BS, there may occur a problem that, because an SCH of an adjacent cell acts as interference, the three-time repetition pattern is broken, thus causing a difficulty in performing timing synchronization.
Also, the SCH of the related art uses a sequence of the same length as that of the number of subcarriers allocated to one SCH. An IEEE 802.16e system of the related art uses 114 sequences to distinguish all of 114 BSs. For example when a length of IFFT is equal to ‘1024’, a length of each sequence is equal to ‘284’ which is the number of subcarriers allocated to one SCH. At this time, an MS determines correlation values between a received SCH signal and the 114 sequences previously possessed, and acquires a cell ID.
An IEEE 802.16m system, a system evolving from the IEEE 802.16e system of the related art, requires more cell IDs than the IEEE 802.16e system of the related art to support a Femto cell. Also, even the number of sequences of an SCH symbol for transmitting a cell ID is increased in proportion to the number of cell IDs. At this time, in general, a correlation characteristic between sequences is deteriorated and thus cell ID detection performance is degraded. Also, a Peak to Average Power Ratio (PAPR) of the sequence increases and thus a margin capable of boosting a transmit power of an SCH decreases.
Also, the IEEE 802.16m system may require that an SCH include supplementary information other than cell ID information for transmission. An example of the supplementary information may be a BS type, sector or segment information, a Fast Fourier Transform (FFT) size, a system bandwidth, etc.
Compared to the IEEE 802.16e system of the related art, the IEEE 802.16m system uses two different types of different symbols (i.e., a Primary SCH (P-SCH) and an S-SCH) to meet additional requirements of the increasing number of cell IDs, supplementary information transmission, etc. At this time, the IEEE 802.16m P-SCH and S-SCH have to divide and perforin a function of an IEEE 802.16e SCH and new functions required by an IEEE 802.16 in SCH. Accordingly, there is a need to define new functions to be carried out by each of the IEEE 802.16m P-SCH and S-SCH. Also, there is a need for a definition of a method for mapping between a sequence of each of the IEEE 802.16m P-SCH and S-SCH and subcarriers.